Advancements on dense-array seismology: A detailed analysis of the ambient noise field recorded in Long Beach, California

Abstract:

With the development of portable and inexpensive seismic instrumentation, the deployment of dense geophone surveys has become a common practice in the field of crustal geophysics. In this study, we use passive data recorded with three ultra-dense geophone arrays in Long Beach, CA, to investigate different features of the local ambient noise field and characterize some of its propagation properties. Examples of wave propagation across the arrays is shown below. In particular, we will show evidence for the occurrence of strong surface wave higher-mode interference and present a mode-modeling approach that we developed to make reliable multi-mode dispersion measurements. In addition to mapping traveltime variations across the survey, our technique allows us to detect large variability in the relative amplitude of the different surface wave modes, which can then be used as new tool to constrain other aspects of the seismic structure such as the depth of the basin. We will also show the existence of strong body wave energy in the correlated wavefields and present a new gradient-based double-beamforming method that allows us to optimally extract refracted P-waves propagating through different sections of the array. The joint analysis of multimode surface waves and high-frequency body waves ultimately allow us to prove the fine-scale velocity structure beneath the survey and generate velocity maps with an unprecedented resolution. Finally, we will outline the limitations of our methodologies and discuss on how they can be extended for use on future arrays as standard dense-array analysis tools.